The present disclosure relates to a control device and method for a turbocharged engine.
Japanese Unexamined Patent Publication No. 2013-133762 discloses an example of a turbocharged engine. Specifically, this engine disclosed in Japanese Unexamined Patent Publication No. 2013-133762 includes: an exhaust passage that is connected to a combustion chamber; a first turbocharger (small turbocharger) that includes a first turbine (small turbine) disposed in that exhaust passage; and a second turbocharger (large turbocharger) that includes a second turbine (large turbine) disposed downstream of the first turbine in the exhaust passage.
The exhaust passage according to the above-described Japanese Unexamined Patent Publication No. 2013-133762 has a second supercharging passage (bypass passage) that bypasses the first turbine and reaches the second turbine, separately from a passage that reaches the second turbine via the first turbine, and a regulating valve that can open/close the second supercharging passage.
Furthermore, the engine disclosed in the above-described Japanese Unexamined Patent Publication No. 2013-133762 has its regulating valve opened or closed in accordance with its operating state so that a first supercharging region (first region) that reduces a gas flow through the second supercharging passage and a second supercharging region (second region) that allows the gas flow through the second supercharging passage are used differently.
For example, in the first supercharging region, the second supercharging passage is blocked by closing the regulating valve. This allows exhaust gas introduced into the exhaust passage to pass through the first turbine and then reach the second turbine without bypassing the first turbine. Thus, the first turbocharger is mainly operated.
On the other hand, in the second supercharging region, the second supercharging passage is released by opening the regulating valve. This allows a large portion of gas introduced into the exhaust passage to bypass the first turbine and reach the second turbine. Thus, the second turbocharger is mainly operated.
In the engine described in the above-described Japanese Unexamined Patent Publication No. 2013-133762, a shift is made from one of the aforementioned first and second supercharging regions to the other one depending on an operating state of the engine. In recent years, such a shift between regions has been desired to be smoothly performed to reduce pump loss, for example.
In view of the foregoing background, it is therefore an object of the present disclosure to provide a turbocharged engine that achieves a smooth shift between an operating region in which the first turbocharger is mainly operated and an operating region in which the second turbocharger is mainly operated.